It is generally desirable for projection system to produce high quality images while being compact and inexpensive. In prior-art transmissive systems using polysilicon liquid-crystal panels, high contrast is achieved by situating each panel between crossed sheet polarizers. Color management beam splitters/combiners and other optics are external to these units, insuring that polarization is not contaminated and contrast degraded.
In projection systems using reflective panels, such as liquid crystal on silicon (LCoS) displays, beam routing is frequently accomplished using polarizing beam splitters FBSs). PBSs are four-port devices, comprising an input port, an output port, and a reflective panel port, with the fourth port typically unused. The behavior of thin-film FBS cubes, for instance, is such that s-polarized light is reflected, while orthogonal, p-polarized light is substantially transmitted.